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Funding Acknowledgements Type of funding sources: None. Introduction Scar burden in atrial fibrillation (AF) can be overestimated due to many factors. Scar burden has prognostic value and substrates considered for ablation by some electrophysiologists. We compared left atrial (LA) scar voltage in AF to sinus rhythm (SR) using voltage histogram analysis (VHA) of those undergoing pulmonary vein isolation (PVI) for persistent AF (PeAF). We believe this is the first study analysing LA scar location in SR and AF using VHA. Methods We retrospectively analysed 120 anatomical segments (AS) and whole LA voltages (N= 10 patients, mean age 68 ± 7, 4 females) in SR and AF. Fast anatomical maps (FAM) were grouped into 6 AS in AF and SR: Anterior, Posterior, Roof, Floor, Septal and Lateral AS, which were analysed via VHA (Figure 1) in 10 voltage ranges between 0mV-0.5mV. Total LA area in each voltage aliquot was recorded in SR and AF, taking diseased LA as 0.2-0.5mV and dense LA scar as 1000 voltage points in both rhythms and uniform procedure involving initial mapping in AF then remapping in SR after PVI. Statistical analyses conducted with IBM SPSS v.26. Results Total LA scar burden was greater in AF (Mean 142.76 mm², SD ± 138.78mm²) than SR (Mean 109mm², SD ± 107.8mm²), p= Conclusions AF was associated with higher scar burden in the Roof, Anterior, Lateral and Posterior AS. Dense LA scar (≤ 0.2mV) on the Posterior AS was significantly higher in AF, while in other AS was comparable to SR. Mapping substrate in AF, especially the posterior wall, may be misleading as scar burden may be overestimated when compared to SR. Table 1Voltage< 0.02 mV (mean area ± SD mm2)0.2-0.5mV (mean area mm2)RhythmSRAFp-valueSRAFp-valueEntire LA115.89 ± 113.61143.41 ± 144.230.02*105.78 ± 103.73144.00 ± 135.24

Introduction Ablation Index (AI) is a novel catheter-based parameter developed to improve and increase efficacy and safety of Pulmonary Vein Isolations (PVI) in the treatment of Atrial Fibrillation (AF). This method involves the incorporation of contact force, time and power in deliverance of ablation lines. The aim of this study is to evaluate the impact of AI on the AF free burden over one-year post redo ablation for persistent AF. The study evaluates a secondary endpoint in medical management escalations post procedure. Methods A retrospective single centre study of 39 patients (Mater Private Hospital, Dublin) who had redo ablations for persistent AF between the period of Jan 2016 till December 2019. We evaluated and analysed the efficacy of AI on redo PVIs in comparison to conventional established ablation techniques. Both groups were matched for age, gender, and duration of PeAF. Group 1 (17 patients) was the AI group with ablation index intervention and group 2 was the contact force (CF) -guided group with 22 patients. Each cardiologist carried out a minimum of 100 ablations per year to confirm the uniformity of performance. Patient follow-up data was analysed at staged intervals for one-year post procedure. Inclusion criteria are: Patients must have had one prior PVI procedure for PeAF, complete follow up data available and all procedures completed using the Carto 3D Mapping System. Statistical analysis was carried out using SPSS (IBM SPSS Statistic Version 26). A Kaplan Meier graph was generated to evaluate the AF free interval. All continuous variables were expressed as the mean ± SD and Students T Test to was applied to give the significant differences for continuous variables. Results Patient characteristics are demonstrated in Table 1. There were no significant differences in age, sex, weight, height, CHADVASC or anti-arrythmia agents, which indicated similar patient profile in each cohort. Pulmonary vein reconnections at redo PVI procedure were comparable at 100% in the AI group and 86% in the CF group (p = 0.1). Freedom from AF burden was (mean 8.72 ± 4.33 months) in CF group Versus (mean 9.35 ± 4.1 months) in AI guided ablation (p=0.71) (figure 1). The AI group demonstrated greater numbers of patients in whom antiarrhythmic therapy could be deescalated over one year (AI, n=11/17, 65% Vs. CF, n=2/22, 9% with p=0.01) while fewer patients underwent escalation of their antiarrhythmic therapy (AI n=2/17, 12% vs CF n=6/22, 27% p= 0.03) (table 1). Conclusion This is the first study to analyse the outcomes of ablation index on repeat PVI procedures. Despite no significant difference in AF recurrence outcomes demonstrated in this study, there was a significant difference in the medical de-escalation in favour of the use of the AI over the short term follow up period. This may reflect effective ablation lesions. A longer-term analysis would be recommended to determine the efficacy of AI use in PeAF redo procedures.